## Description 

This mode is similar to the mode before `consensus_batched_block_sync`
is added. The difference is that missing ancestor blocks are deduped and
selected randomly, to reduce duplicated blocks fetched.

## Test plan 

CI
PT

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## Release notes

Check each box that your changes affect. If none of the boxes relate to
your changes, release notes aren't required.

For each box you select, include information after the relevant heading
that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] gRPC:
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK: